docs: def addExtensionArg(self, namespace, key, value):
          """Add an extension argument to this OpenID authentication
          request.
  
          Use caution when adding arguments, because they will be
          URL-escaped and appended to the redirect URL, which can easily
          get quite long.
  
          @param namespace: The namespace for the extension. For
              example, the simple registration extension uses the
              namespace C{sreg}.
  
          @type namespace: str
  
          @param key: The key within the extension namespace. For
              example, the nickname field in the simple registration
              extension's key is C{nickname}.
  
          @type key: str
  
          @param value: The value to provide to the server for this
              argument.
  
          @type value: str
          """
          self.message.setArg(namespace, key, value)